The process for the production of a Portland-type cement from blast furnace slag in a molten bath reactor requires additions of calcium carbonate or calcium oxide to the slag.
An object of this invention is to minimise the requirement for additional energy for calcination by utilizing the energy of the hot off-gases that leave the molten bath reactor.
Calcination of, for example, limestone, is performed in a variety of furnaces: vertical shaft kilns, rotary kilns, rotary hearth kilns, twin and multiple shaft regenerative kilns, fluidised beds and entrained flow reactors.
Vertical shaft kilns produce a lower quality of lime compared with other types of kilns, and can only accept feed of no less than about 50 mm in size. Smaller sized feed results in a packed bed of insufficient porosity. Nevertheless, the vertical shaft kiln is a very energy efficient kiln.
Rotary kilns are generally able to accept almost any sized feed, down to about 1 mm in size. However, to obtain good energy efficiency it is necessary to incorporate preheaters in combination with the kilns. This combination suffers from twin disadvantages of having high capital and maintenance costs.
Twin and multiple shaft regenerative kilns offer good energy efficiency, since the waste gas from the calcining kiln is used to preheat other kilns. This sequence is continued around the kilns--successively preheating then calcining in each kiln. However, this type of kiln is not suitable for treating fine sized materials.
Fluidised bed calciners are generally able to treat finer sized materials than shaft furnaces. However, control of particle size distribution is important, and large quantities of hot pressurised gas are needed for the fluidisation.
Unfortunately, non of these processes can be adapted so that the hot off-gases from a molten bath reactor, which usually contain sticky and/or molten material, can be used as the energy source of calcination.
In U.S. Pat. No. 3,022,989 (Union Commerce Bank) a process for manufacturing hydraulic cements is disclosed. In this process there is a fluidised bed of particles, in which gases are combusted. A certain amount of stickiness of particles results, but the degree of stickiness is controlled so as to be insufficient to cause large particles to become attached to one another, and agglomeration of the fluidised mass is thereby prevented.
Unfortunately this process requires a fuel to be combusted to provide the high temperatures. It is unlikely that hot off-gases from a molten bath reactor, which contain sticky grains and/or molten droplets could be used to provide the high temperatures. The process operates at a high solids loading, and as a consequence requires that there be a large pressure drop as gases flow through the bed. Furthermore, by virtue of the well-mixed nature of fluidised beds it is not possible to have very large differences between the temperatures of the hottest and coldest particles in the bed.
In patent application No. P251515 IVa/12g in the Federal Republic of Germany by Polysius GmbH, an apparatus is disclosed for the combustion, calcination and sintering of a pulverized or granulated product in suspension, and to the operating procedure for such an apparatus. The apparatus seems to be a modified cyclone with a flattened or `pancake` cross-section.
The drawings in the Polysius specification are confusing and seem to contradict the specification, since the paths of the treatment gas, the pulverized or granulated material to be treated and the carburant are drawn in a haphazard manner. The best conclusion seems to be that entrainment of the material to be treated is transitory only, the treated material being separated almost immediately from the treatment gas.
No attempt seems to be made to minimise contact of the particles with each other, with a consequent danger of agglomeration. In addition, the particles travel to the wall of the treatment chamber for removal, with a danger in this case of accretion to the wall.
Furthermore, the pulverized or granulated material seems to be injected counterflow to the air. In addition carburant is burned in the relatively small space where the particles and gas are travelling counterflow. This must lead to severe turbulence, with particles contacting each other and the wall of the treatment chamber.
Finally, the apparatus and process of Polysius do not make use of the sensible heat from off-gases, rather heat must be supplied by burning fuel.